Add Performance Monitor and ability to temporarly disable heavy layers

On heavy projects it would help a lot to have a way to monitor computation time of layers to find bottlenecks in projects (as in Substnace Designer or Houdini for example).

And you could also go further and add an ability to temporarly toggle computation of those heavy layers off (detected automaticlly or by user's choice).
So, for example you press a hotkey, it freezes computation of those heavy layers, which allows you to paint masks, add/move decals, etc. freely without a lag and then press hotkey back again to recompute and see the final look.

As the project grows it becomes more painfull and painfull to manipulate stuff. And this seems to be a good solution for this.